Method of introducing hard phases into metallic matrices



July 14, 1959 E. GREGORY METHOD OF INTRODUCING HARD PHASES INTO METALLIC MATRICES Filed Oct. 11, 1956 M s WWW .m m. a n w 5? N 0 o .RM 02/ 6 5 3 v E25 l wiw a w kmwero/o v fl mw 8 AC w w w n W w m fE'MPEE/I rams- "6 INVENTOR.

Ua wd States PM? I F- INTRODUCING IiARD -PHASES INTO METALLICMATRICES Erie "Gregory, Hastings on Hudson, 'N.Y., as'sigito'r to corporation of New York I Appli cation October 11,1956 Serial No. 615,336

I 11 Claims. c1. 75-206) I -Sintei'cast Corpoi-ation of America, Yonkers, N.Y., a

The present invention relates to the production of a reinforced metal product and, more particularly, to a method of'iutroducing a hard refractory oxide phase into a ductile metal matrix. I

It isdcnown that high temperature strength properties of-certain ductile metals can be improved by the addition to the metals of a discrete hard phase, -e.g. Al O ,"which increases the metals resistance to deformation and en- .abl es it to sustain highstress atelevated temperatures for prolonged periods of time Wroughtaluminum products produced from fine aluminum powders exhibit such improved properties in comparison to-aluminum products madeby conventional casting. This is because aluminumpowders are generally characterizedby a surance'to applied stress at elevated temperatures. The

.disperse hard'phase acts as a deterrent tOrecrystallization and grain growth and thusinh'ibits'ordecreases the rnetals tendency to weaken and soften at elevated temperatures.

There have been recent tr'ends'to employfthis disperse phase hardening .mechanisni in, s. ,ni 'g heatresistant alloys, for example an alloy containing 80% nickel and 20% chromium. A given amount of hard phase, e.g. titanium carbide powder, is added to the fc'n'egoing' type alloy in the powder form and the two mechanically mixed to obtain a uniform distribution of one in the other. The mixture is consolidated into a'compact and then hot worked at an elevated temperature into a desired shape. Thefine dispersion of the carbide stiifens the matrix of the alloy and confers additional re sistance to deformation at high stresses at elevated 'temperatures. In order to obtain full benefit, the hard phase must be substantially insoluble in the matrix of "the, alloy, otherwise the alloy will lose stiffness and weaken due 'ot the solution of the hard phase in thematrix metal. a

One of the disadvantages of the foregoing method of introducing hard phases into the matrix -was the tendency for mechanically mixed materials to segregate." Because of this efiect, it was not always consistently pos sible to maintain a perfect mixture; Even when the powders were blended under the best possible mixing prevailing in buildings containing 'large pressesgreciprocating vacuum pumps and other heavy industrial machinery andequipment, the powders would tend to segregate due to marked differences in densities and', unl ess the po wders were remixed, the final product would not always exhibit the desired properties due to non-uniform distribution of the slip-inhibiting phase in the final product. I

2,894,838 Patented July lfl, 9

Thepresentfinvention overcomes the foregoingidifliculties'by providing a method for insuring uniformity of the finalproduct' by enabling the production of- "a metal powder with a slip -inhibiting phase associated'Withit. It is, therefore, an object of the present inventionto provide a method of'producing metal powder having a hard phase associated therewith prior toconsolidating the powder into a solid shape, thereby insuring a uniform distribution of said hard phase throughout the final product. Other objects will more clearly appear fromthe-following description taken in conjunction with the accompartying drawing which compares deforming stresses-at various temperatures of a product of the invention with a similar product produced by conventional methods' The introduction of the'hard phase is achievedby the liquid phase sintering ofa refractory oxide-yielding composite metal powder with a solid ductile matrix metal powder of melting point above that of the metal-in the composite powder, preferably above 750 C. i By refractory oxide-yielding powder is meant a metal 'powderhaving a propensity of forming a refractory oxide coating of high melting point, the metalunderne'aththe 'coating 'haying a melting point below that of said matrix metal'powder', preferably below about 900 C. and more preferably within-the range of about 600 C. to 900C.

'In'carrying the-invention into practice, a matrix metal powder is uniformly mixed and blended with suflicient refractory oxide-yielding composite metal powder characterized by a high melting point refractory oxide coatin'g. The-mixture is tamped into a confining mold,"e.g. a steel tube, and then subjected toliquidphase'sintering under reducing conditions at a temperature'above powder is' then treated like any metal powder in forming a consolidated product. For example, the powder may be hot pressed into an ingot under neutral or reducing conditions and thereafter hot extruded at an elevated'temperature into a desired shape. The resulting product will be characterized by a metal matrixhaving dispersed therethrough .finely divided particles of refractory oxide and willcxhibit improved resistance to creep above'room temperature.

An example of a refractory oxide-yielding composite powder isaluminum. .Finely divided aluminum powder when oxidized contains a refractory oxide surface coating and, depending on how the powder isfproduced, may

contain oxygen (as Al O in amounts ranging from about 0.2'to .15%' by weight of the powder. Atomized ,aluminumpo wder generally exhibits a low oxygencon- .tent. (for example inthe neighborhoodof 0.5%) while very fine flake powder (e.g. minus 325 mesh, U.S. Standard screen size) can be produced having a higher oxygen content ranging as high as 15% by weight andeven a little higher. This composite powder is ideal because first the melting point of the metal (about 660 C.) makes it desirable for liquid phase sintering with a solid matrix ,me'tal powder; second, the aluminum oxide is hard, stable and substantially insoluble in the matrix metal at elevated temperatures and, therefore, makes an excellent slipinbitor; and, third, the oxide coating hasa'relatively melting point (over 2000 C.).

Magnesium metal powder is another suitable material.

Like aluminum, it forms a hard, stable "oxide coating *(Mg0) whieh has-a-high'melting point-(a'bout 2500 C;

compared to the much lower melting point of the metal itself (about 650 C.) and similarly is substantially insoluble in the matrix metal..

. J; The rareearth metals such as cerium, lanthanurm neo- 'dymium, etc. could also be used, if desired, since they hayje, melting points falling within the, range of about ,600" C. to 900 C. Cerium has a relatively'low melting point in the neighborhood of 600 C., lanthanum in the neighborhood of about 825 C., and neodymium in the neighborhood of 840 C. Their oxides on the other hand melt at a much higher temperature of over 1600'? C. It will be apparent from the foregoing that. the liquid phase sintering materials (i.e. Al, Mg, Ce, etclyareall characterized by melting points below 950 C. while their respective oxides are hard and stable and have very high melting points in excess of 1600 C. n n ,ijThe matrix metals on the other hand have melting points above those of the refractory oxide-yielding metal,

preferably above 900 C. Examples of matrix metals for the purposes of this invention are. silver, gold, copper, platinum, palladium, nickel, cobalt, iron, and alloys based on one or more of these metals, etc. Such matrix metals may be selected broadly from the copper group, platinum group and iron group metals and their alloys. Other matrix metals may include tungsten, molybdenum, columbium etc. a q 7 l The matrix metals are preferably definedbroadly-as those whose oxides have a negative free energy, of formaconsideration as it enables the carrying out of certain preferred embodiments of the invention to be described later.

T As illustrative of the invention, the following examples are given:

\ Example I Fine flake copper powder (minus 200 mesh) and 8% by weight of very fine flake aluminum powder (minus 325 mesh) having an oxygen content of about 15% by weight of the aluminum powder are mechanically blended together and'the mixture packed in a steel tube. The mixture is then heated in hydrogen to 900 C. and sub- ,jected to liquid phase sintering for one hour. The result- Sing fritted powderwhich has a yellowish, bronze color is then pulverized, cold compacted, thereafter heated to 1000 C. in nitrogen, placed in a cold die and immediately 'hot pressed at 30 tons per square inch. The hot pressed billet is thenmachined, heated to 1000 C. and then hot i extruded at a pressure .of tons per square inch and an extrusion ratio of 20 to 1 to form an extruded aluminum bronze alloy containing about 2.5% by weightof Al- 03 as a slip inhibitor.

The drawing shows a comparison of propertiesof aluminum bronze produced in accordance with Example I (note curves. (1) and (2)), with conventional cast or wrought aluminum bronze (3). The extruded material produced in accordance with thisinvention (1) was tested at various temperatures under stresses where appreciable deformation occurred. It will be noted that the alloy of the invention (1) can sustain higher loads be'fore deforming appreciably at temperatures up to 500 C. than a confventional cast and wrought aluminum bronze alloy containing about 10% aluminum. Even when the invention alloy is soaked for a long period of 16 hours at 900 C.,

note curve (2), it still sustains higher loads before ap- 'preciably deforming. C

n v Example II ..The invention is also applicable to the production of .a'ge-hardenable, mas e iumsomain na ni kel a y ,1

producing a Ni-Mg alloy containing about 1% magnesium, powder nickel of minus 200 mesh is mixed with powder magnesium of minus 325 mesh containing about 10% by weight of oxygen as a surface layer of MgO. The mixed powders are tamped in a confining chamber and then heated in a reducingatmosphere and subjected to liquid phase sintering at a temperature of about 700 C. for about 1 hour (it is preferred that the temperature not exceed 700 C. or 750 C. in iew'of the relatively high vapor pressure of magnesium at high temperatures). The resulting frittedl an alloyed powder is pulverized, cold compacted and thereafter heated to atemperature of about 800 C. in an inert atmosphere, placed in a cold die and immediately hot pressed at a pressure of 30'to 40 tons per square inch. The resulting billet is machined, sheathed airtight in an iron container, heated to 1000" C., and then hot extruded at a pressure of about 50 tons per square inch and an extrusion ratio of about 20 to 1 to form an extruded nickel alloy containing some magnesium in solid solution and some magnesiumas a disperse hard phase of MgO. An addition of 1% magnesium having an oxygen content of about 10% will result in a nickel alloy containing MgO in the neighborhood of .about 0.25% bylweight. I

r Examplelllm Similarly, informing a nickel-aluminum age-hardening alloy, e.g. an alloy containing about between 4 to 5% "aluminum, somewhat the same'procedure may be employed. In producing the alloy, about 5% of aluminum powder containing 15% oxygen is mechanically mixed with nickel powder (minus 325 mesh) and the mixture tamped into amold. The mixture is then subjected to liquid phase 'sinterin'g ata temperature of about 1000" 0., whereby substantially all of the aluminum is alloyed with the nickel, the oxide shellremaining becoming associated with the alloy particles. A frit results which is pulverizedto pass through 200 mesh; The pulverized mixture is then cold compacted and thereafter heated to a temperature ofabout 1000 C. in a reducing atmosphere and immediately placed in a cold die and hot compacted at about 30 to' 40 tons'per square inch, machined into an extrusion 'slugfslieathed airtightin an iron container and then finally hot extruded at about 75 tons per square inch at a temperature of about 1000 to 1050 C. and an extrusion ratio of' 18 to'l to formIa nickel-aluminum alloy shape characterized by afine dispersion of} approximately 1.5% A1 0 by weight throughout.

Heat resistant alloys can likewise be similarly treated.

For example, an alloy powder comprising nickel and 2.0% chromium can be treated as above.

Likewise heat resistant cobalt-base and iron-base alloy powders may be also treated. i i i Where ahigh oxide content is desired in the final product, for example. an oxide content of about 5% or higher of A1 0 this can be achieved by starting with a partlally oxidized matrix metal powder which is reacted -with aluminum or similar powder to produce the desired refractory oxide. In this respect it is important as indicatedhereinbefore that the negative free energy of formation of the matrix metal oxide at about 25 I C. be less than 70,000 calories per gram atom of exygen, while that for therefractory oxide-yielding metal be at least greater ,than 90,000 and preferably greater than 120,000 calories per gram atom. Thus, nickel powder partially oxidized to; an oxygen contentpf about 5% would be subjected to liquid phase sint'ering at about 1000 C. with flake aluminum powder containing about 15% oxygen. The .mckel oxide (NiO) having a negative free energy of read with liquid aluminum and bereduced by it to formation of about 51,700 calories at about 25 C. will nickel, the aluminumin turn being oxidized to A1 0 since the negative free energy offormation of aluminum oxide at about 2.5 C. is'of theorder of 125,590 calories per gram atom. Thus. the A formed by reactiqn 5 augments'the Al -already contained in the flake alumi'-' num powder. I l

Assuming 92- parts of nickel powder containing" 5% oxygen is reacted at 1000- C. with 8 partsaluminum flake powder containing]- 1'5% oxygen, and assuming the liquid phase reaction between aluminum and NiO goes to completion, the mixture will contain about 12.35% by weight of- Al O the balance comprising about 87.4% nickel with'a small amount of aluminum- (about 0.25%) alloyed therewith. It'is apparent from the foregoing that many variations can be resorted to in utilizing the advantages of' the invention.

As illustrative of thetypeof ductile copper group, platinum group and iron group alloys that can be treated in accordance with the invention, the following is given:

Examples of copper group alloys are: 95% copper and 5% zinc; 90% copper and 10% zinc; 60% copper and 40% zinc; 71% copper; 28% zinc and11% tin; 65% cop per, 17% zinc and'l8% nickel; 90% silver and 10% copper; up to 12% nickel and the balance silver; 70% goldand the balance palladium; 69% gold, 25% silver and 6% platinum, etc-Q The copper group matrix metals and their alloys may be liquid phase sintered with refractory oxide-yielding metals as, aluminum at a temperature range above the melting point of the oxide-yielding metal, e g. from about 700 C. to below the melting point of the matrix metal, usually within the range of 700 C. to 1000 C.

Examples of iron group alloys include certain steels; 64% iron and 36% nickel; 31% nickel; 4' to 6% cobalt, and the, balance iron; 54% iron and 46% nickel; 99% nickel and the balance cobalt; 68% nickel and 32% copper, etc. Heat resistantalloys based on the iron group metals can also be treated including stainless steels, and-heat resistant nickelbase and' eobalt-base alloys. The liquid phase sintering temperatures of, the foregoing matrix metals with the refractory oxide-yielding metal may similarly range from 700 f" C; to below the melting point of the'lmatrix metals e.g. from 700 C; to 1200 C,

Examples of platinum group alloys are as follows: platinum-rhodium alloys containing up to 50% rhodium; platinum-iridium containing up to30% iridium; platinumnickel containing up to 601 10%: nickel; platinumrpalladirim-ruthenium containing 77% to l0 platinum, 13% to 88% palladium, and 10% to 2% ruthenium; alloys of palladium'rruthenimn containing up to 8% ruthenium; 60% palladium and 40% silver, etc. These alloys with refractory oxide-forming metalwo'uld. be treated similarly as the copper group alloys. I p

When employing aluminum. powder as the refractory oxide-yielding metal, it is preferred that the powder be non-leafing and have a particle size of less than 200 mesh and, preferably, less than 325 mesh. Other metal powders similar to aluminum should likewise be preferably maintained at minus 200.0r 325 mesh. I,

The oxygen content of the refractory oxide-yielding powder may range broadly from about 0.2% to 15% by weight and may be liquid phase sintered with the matrix metal, over the temperature range of 700.to 1200 C. In producing reinforced metal' products, the composite powder produced in accordance with the invention is first consolidated before it is. hot worked or extruded. It is preferred that the mixture be consolidated to an apparent density of at least about 65 preferably as near as 9.0% as possible, before. it is hot worked. While general it'woiild be preferred that themixture be cold pressed, it will be appreciated that hot pressing can also be employed. In producing a compact of at least 65 apparent density by cold pressing, the pressure applied may range from 15 to 50. tons per square inch. When hp pressing is, employed, the temperature will usually range from 650, C. to 1250 C. at pressures ranging up to about 40-tons per square inch, l ower pressures being empleyed at higher temperature levels. .Where high compac'ting pressure in conjunction with high temperature is employed the billet is heatedseparately, placed in a cold die and immediately hot compacted. When hot pressing is conducted, it should be carried out under protective conditions, e.g. in a reducing atmosphere, an inert atmosphere or even at subatmospheric pressure.

Likewise when the compact is extruded, the conditions also should be protective to the materials forming the compact. Encasing the compact in an airtight, evacuated, welded container, for example a sheath. of iron or nickel, is one method of protecting the materials in the compact from oxidation, etc. Depending on the situation, extrusion pressure may range from about 40 to 250 tons per square inch over a temperature range of about 800 C. to 1250 C: for extrusion ratio, ranging from about l4to1and20 to 1. I

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I' claim:

1. A method of intnoducinga hard-slip inhibiting phase into a ductile matrix metal, an oxide of; which has a negative free energy of formation-at about 25 C. of below 70,000 calories per gram atom of oxygen which comprises-providing a powder of said ductile matrix metal of melting point above 750 C. having mixed therewith a refractory oxide-yielding metal powder of melting point belowthat of the matrix metal and having a refractory oxide coating which substantially stable at elevated temperatures and has a negative free energy offormation at about 25 C. of over 90,000'calories per gram atom of oxygen, subjecting said mixed powder to liquid phase sintering at an elevated temperature above the melting point of said refractory oxide-yielding metal and below the melting point of said matrix metal, whereby the refractory oxide-yielding metal melts, escapes from beneath its oxide shell andalloys with said matrix metal powder, the refractory oxide shell becoming associated with said alloy powder, consolidating said powder to a solid compact and hot forming said compact to a desired shape, whereby said shape is characterized by a uniform dispersion throughout of particles of said hard refractory oxide. 1

2. A method of introducing a hardslip inhibiting phase into a ductile matrix metal an oxide of whichhas a negative free energy of formation at about 25 C. of below 70,000 calories per gram atom 'of' oxygen which comprises providing a powder of said ductile matrix metal of melting point above 750 C. having mixed therewith up to about 10% by weight of at least one refractory oxide-yielding metal powder selected from the group consisting of aluminum, magnesium, cerium, lanthanum and neodymium of melting point below that of the matrix metal and having an oxygen content of about 0.2% to 15% in the form of a refractory oxide coating based on said oxide-yielding metal which is substantially stable at elevated temperatures and has a negative free energy of formation at about 25 C. of over 90,000 calories per gram atom of oxygen, subjecting said mixed powder to liquid phase sintering at an elevated temperature above the melting point of said refractory oxide-yielding metal and below the melting point of said ductile matrix metal within the range of about 700 to l200 Q, whereby the refractory oxide-yielding metal melts, escapes from beneath its oxide shell and alloys with said matrix metal powder, the refractory oxide shell becoming associated with said alloy powder, comminuting said treated powder, consolidating said powder to a solid compact and hot forming said compact to a desired shape, whereby said shape is characterized by a uniform dispersion throughout of fin'e particles of a hard refractory oxide.

3. A method of introducing a hard slip inhibiting phase into a ductile matrix metal of melting point above 750 C., an oxide of which has'a negative free energy of forma-' tion at about 25 9 C. of below 70,000 calories per gram atom of oxygen, which comprises providing a powder of said matrix metal having mixed therewith up to about 10% by weight of finely divided aluminum powder characterized by an oxygen content of about 0.2% to in the form of a refractory aluminum oxide coating whose negative free energy at about 25 C. is over 90,000 calories per gram atom of oxygen, subjecting said mixed powder to liquid phase sintering at an elevated temperature above the melting point of aluminum within the range of about 700 to 1200 C., whereby the aluminum melts, escapes from beneath its oxide coating and alloys with said matrix metal powder, the refractory aluminum oxide coating becoming associated with the particles of said alloy powder, comminutiug said treated powder, con solidating said powder to a solid compact and hot forming said compact to a desired shape, whereby said shape is characterized by a uniform dispersion throughout of fine particles of said aluminum oxide.

4. A method of introducing a hard slip inhibiting phase into a ductile matrix metal an oxide of which has a negative free energy at about 25 C. or below 70,000 calories per gram atom of oxygen, which comprises providing a powder of said matrix metal of melting point above 750 C. with an oxidized surface and having mixed therewith refractory oxide-yielding metal powder of melting point below that of the matrix metal and having an oxygen content ofjabout 0.2% to 15 by weight in the form of a refractory oxide coate ing based on said oxide-yielding metal which is subtantially stable at elevated temperatures and has a negative free energy of formation at about 25 C. of over 90,000 calories per. gram atom of oxygen, subjecting said mixed powder to liquid phase sintering at anelevated temperature above the melting point ofsaid refractory oxide-yielding metal and below the melting point of said matrix metal, whereby the refractory oxide-yielding metal melts, escapes from beneath its oxide shell and reacts with the oxide coating on said matrix metal powder, reduces it with the consequent formation of more refractory oxide, the newly formed oxide together with the original refractory oxide becoming associated with said alloy powder, comminuting said treated powder, consolidating said powder to a solid compact and hot forming said compact to a de* sired shape, whereby said shape is characterized by a uniform dispersion throughout of fine particles of a hard refractory oxide.

5. A method of introducing a hard slip inhibiting phase into a ductile matrix metal of melting point above 750 C., an oxide of which has a negative free energy at about 25 C. of below 70,000 calories per gram atom of oxygen, which comprises providing a powder of said matrix metal with an oxidized surface and having mixed therewith at least one refractory oxide-yielding metal powder selected from the group consisting of aluminum, magnesium, cerium, lanthanum and neodymium of melting point below that of the matrix metal and having an oxygen content of about 0.2% to 15% by weight in the form of a refractory oxide coating based on said oxide-yielding metal which is substantially stable at elevated temperatures and has a negative free energy of formation at about 25 C. of over 90,000 calories per gram atom of oxygen, subjecting said powder to liquid phase sintering at an elevated temperature above the melting point of said refractory oxide-yielding metal and below the melting point of said matrix metal within the range of about 700 C. to 1200 C., whereby the refractory oxide-yielding metal melts, escapes from beneath its oxide shell, and reacts with the oxide coating on said matrix metal powder, reduces it with the consequent formation of more refractory oxide, the

newly. formed oxide together with the original re fractory oxide becoming associated with the particles of said alloy powder, comminuting' said treated powder, consolidating said powder toa solidcompact and hot forming said compact to a desiredrshape, whereby said shape is characterized by a uniform dispersion throughout of fine'particles of a hard refractory oxide.

6, A method of introducing a hard slip inhibiting phase into a ductile matrix metal an oxide of which has a negative free energy of formation at about 25 C. of below about 70,000 calories per gram atom of oxygen, which comprises providing a powder of said matrix metal of melting point above 750 C. with an oxidized surface having mixed therewith finely divided aluminum powder characterized by an oxygen content of about 0.2% to 15% inthe form of a refractory aluminum oxide coatingwhose negative free energy of formation at about.25 C. is over 90,000 calories per gram atom of oxygen, subjecting said mixed powder to liquid phase sintering at an elevated temperature above the melting point of aluminum and below the melting point of said matrix metal within the range of. about700 C. to 1200 C., whereby the aluminum escapes from beneath its oxide coating and reacts with the oxide coating on said matrix metal powder, reduces it with the consequent formation of new aluminum oxide, the newly formed oxide together with the original aluminum oxide becoming associated with .the particles of said alloy powder, comminuting said treated powder, consolidating said powder to a solid compact and hot forming said compact to a desiredshape, whereby said shape is characterized by a uniform dispersion throughout of fine particles of a hard refractory oxide.

'7. A method of introducing a hard slip inhibiting phase into a ductilematrix metal, powder, an oxide of which has a negative free energy of formation at about 25 C. of below 70,000v calories per gram atom of oxygen, which comprises providing a .powder, of said matrix metal of melting point above 750 C. having mixed therewith a refractory oxide-yielding metal powder of melting point below that of the matrix metal and having an oxygen content of about 0.2% to 15% in the form of a refractory oxide coating which is substantially stable at elevated temperatures and has a negative free energy of formation at about 25 C. of over 90,000 calories per gram atom of oxygen, subjecting said mixed powder to liquid phase; sintering at an elevated temperature above the melting. point of said refractory oxide-yielding metal and below the melting point of said. matrix metal, whereby the refractory oxide-yielding metal melts, escapes from beneath its oxide shell, and alloys with said matrix metal powder with the refractory oxide coating thereafter becoming associated with said resulting alloy powder.

8. A method of introducing a-hard slip inhibiting phase into a ductile matrix metal powder of melting point above 750 C., an oxide of which has a negative free energy of formation at about 25 C., of below 70,000 calories per gram atom of oxygen, which comprises providing a powder of said matrix metal having mixed therewith at least one refractory oxide-yielding metal powder selected from the group consisting of aluminum, magnesium, cerium, lanthanum and neodymium of melting point below that of the matrix metal and having an oxygen content of about 0.2% to 15% by weight in the form of a refractory oxide coating based on said oxide-yielding metal which is substantially stable at elevated temperatures and has a negative free energy of formation at about 25 C. of over 90,000 calories per gram atom of oxygen, subjecting said mixed powder to liquid phase sintering at an elevated temperature above the melting point of said refractory oxide-yielding metal and below the melting'point of said matrix metal within the range of about 700 C. to 1200 C., whereby the refractory oxide-yielding metal melts, 6S-

capes from beneath its oxide shell and alloys with said matrix metal powder with the refractory oxide coating thereafter becoming associated with said resulting alloy powder and then comminuting said powder.

9. A method of introducing a hard slip inhibiting phase into a ductile matrix metal powder of melting point above 750 C., an oxide of which has a negative free energy of formation at about 25 C. of below 70,000 calories per gram atom of oxygen, which comprises providing a powder of said matrix metal having mixed therewith finely divided aluminum powder characterized by an oxygen content of about 0.2% to in the form of a refractory aluminum oxide coating whose negative free energy of formation at about 25 C. is over 90,000 calories per gram atom of oxygen, subjecting said mixed powder to liquid phase sintering at an elevated temperature above the melting point of aluminum within the range of about 700 C. to 1200 C., whereby the aluminum melts, escapes from beneath its oxide coating, and alloys with said matrix metal powder with the refractory aluminum oxide coating thereafter becoming associated with said resulting alloy powder and then comminuting said powder.

10. A method of introducing a hard slip inhibiting phase into a ductile matrix metal powder an oxide of which has a negative free energy at about 25 C. of below 70,000 calories per gram atom of oxygen which comprises providing a powder of said matrix metal of melting point above 750 C. with an oxidized surface and having mixed therewith at least one refractory oxideyielding metal powder selected from the group consisting of aluminum, magnesium, cerium, lanthanum and neodymium of melting point below that of the matrix metal and having an oxygen content of about 0.2% to 15% by weight in the form of a refractory oxide coating based on said oxide-yielding metal which is substantially stable at elevated temperatures and has a negative free energy of formation at about 25 C. of over 90,000 calories per gram atom of oxygen, subjecting said mixed powder to liquid phase sintering at an elevated temperature above the melting point of said refractory oxideyielding metal and below the melting point of said matrix metal, whereby the refractory oxide-yielding metal melts,

escapes from beneath its oxide shell, and reacts with the oxide coating on said matrix metal powder, reduces it with the consequent formation of more refractory oxide, the newly formed oxide together with the original refractory oxide becoming associated with said resulting alloy powder.

11. A method of introducing a hard slip inhibiting phase into a ductile matrix metal powder an oxide of which has a negative free energy at about 25 C., of below 70,000 calories per gram atom of oxygen of melting point above 950 C. which comprises providing a powder of said matrix metal of melting point above 750 C. with an oxidized surface and having mixed therewith finely divided aluminum powder characterized by an oxygen content of about 0.2% to 15% in the form of a refractory aluminum oxide coating whose negative free energy of formation at about 25 C. is over 90,000 calories per gram atom of oxygen, subjecting said mixed powder to liquid phase sintering at an elevated temperature above the melting point of said aluminum within the range of about 700 C. to 1200 C., whereby the aluminum melts, escapes from beneath its oxide coating and reacts with the oxide coating on said matrix metal powder and reduces it with the consequent formation of more refractory oxide, the newly formed refractory oxide together with the original refractory aluminum oxide coating thereafter becoming associated with said resulting alloy powder, and then comminuting said powder.

References Cited in the file of this patent UNITED STATES PATENTS 927,935 Von Bolton July 13, 1909 2,100,537 Conway Nov. 30, 1937 2,404,598 Sachse July 23, 1946 2,657,128 Stern et a1. Oct. 27, 1953 2,657,129 Stern et a1. Oct. 27, 1953 FOREIGN PATENTS 625,364 Great Britain June 27, 1949 160,526 Australia July 17, 1952 730,931 Great Britain June 1, 1955 

1. A METHOD OF INTRODUCING A HARD SILP INHIBTING PHASE INTO A DUCTILE MATRIX METAL, AN OXIDE OF WHICH HAS A NEGATIVE FREE ENERGY OF FORMATION AT ABOUT 25* C. OF BELOW 70,000 CALORIES PER GRAM ATOM OF OXYGEN WHICH COMPRISES PROVIDING A POWDER OF SAID DUCTILE MATRIX METAL OF MELTING POINT ABOVE 750* C. HAVING MIXED THEREWITH A REFRACTORY OXIDE-YIELDING METAL POWDER OF MELTING POINT BELOW THAT OF THE MATRIX METAL AND HAVING A REFRACTORY OXIDE COATING WHICH IS SUBSTANTIALLY STABLE AT ELEVATED TEMPERATURES AND HAS A NEGATIVE FREE ENERGY OF FORMATION AT ABOUT 25* C. OF OVER 90,000 CALORIES PER GRAM ATOM OF OXYGEN, SUBJECTING SAID MIXED POWDER TO LIQUID PHASE SINTERING AT AN ELEVATED TEMPERATURE ABOVE THE MELTING POINT OF SAID REFRACTORY OXIDE-YIELDING METAL AND BELOW THE MELTING POINT OF SAID MATRIX METAL, WHEREBY THE REFRACTORY OXIDE-YIELDING METAL MELTS, ESCAPES FROM BENEATH ITS OXIDE SHELL AND ALLOWYS WITH SAID MATRIX METAL POWDER, THE REFRACTORY OXIDE SHELL BECOMING ASSOCIATED WITH SAID ALLOY POWDER, CONSOLIDATING SAID POWDER TO A SOLID COMPACT AND HOT FORMING SAID COMPACT TO A DESIRED SHAPE, WHEREBY SAID SHAPE IS CHARACTERIZED BY A UNIFORM DISPERSION THROUGHOUT OF PARTICLES OF SAID HARD REFRACTORY OXIDE. 